Human powered surface drive?

Discussion in 'Boat Design' started by clmanges, Aug 26, 2008.

  1. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    Now I see that a lot already has been discussed while a was enjoying a well-deserved sleep.
    Like Rick wrote, there are much more efficient methods to convert muscle power into marine propulsion than a surface drive.

    I've been wrestling with my tunnel drives for half a year now and learned a lot (the hard way). Surface piercing props look good on paper, but in real life a lot is different. The border between water and air is not as sharp as you would expect it to be once the boat starts moving faster. The props lift water in more or less random directions and expel an unpredictable mixture of air and water. On my boat the 2 props produce a -to the eye- single jet, partly in the water and partly above the surface.
    Friction is much lower than that of a submerged prop, so for a given engine output you can use larger props and/or obtain higher rpm. The disadvantage is the very asymmetrical load on the shaft and bearings, causing rumble and vibrations.
    There is an old freighter in this area that supplies water to remote island. It is just a hull with an open cargo hold and a bulkhead to keep the engine room from flooding. When she empty, the prop shaft is well above the water line and it is a spectacular sight to watch the ship from a distance. You don't need to be an engineer to see that this is not very efficient way to move an object.

    In my humble opinion, surface piercing systems are only an option for planing craft and even then they would only be the first choice if there are factors involved like shallow water operation or the need to place the engine close to the stern.
    And planing with a man-powered craft? A few minutes maybe, not more.
     
  2. Guest625101138

    Guest625101138 Previous Member

    CDK
    Your comments interested me so I checked out your project. I am impressed at what you have done. You may not be able to get on the plane (yet) but the fuel saving is impressive and I think today that is a crucial objective.

    This may not be the right place but I would be interested to know how much power you have got left after the props ventilate. The reason I ask is if you believe there is enough grunt to plane. I too learnt about the function of surface props but my project was about two orders of magnitude less complex as photo in earlier post shows.

    Surface props are quite efficient if they are designed for the application. Clearly your existing props are not surface props.

    There may actually be a means of providing proper ducting so you have outboard jets and the ducting would not add a lot of drag at 20kts.

    Rick W
     
  3. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    Thank you for your interest in my project Rick.
    I also don't want to hijack this thread, but the other participants are still in bed or brushing their teeth.

    I should have set my targets a bit lower and will never again plunge into deep water again without proper assessment of time and resources. Building unusual drives, adding a foot to an existing hull, converting two VW turbo diesels and electronically control throttles, gearboxes and cold start devices has been a bit much for and old guy. It was especially the disappointment with the gas guzzling jets that made me change direction. And after that, one thing led to another.

    As to your questions:
    There is at the present approx. 180 hp available at 4500 rpm, which should be enough to obtain stable planing speed. Sacrificing fuel efficiency the output can be increased at least 20%. The old Mercs with 2x 130 (Mercuiser-)hp produced about the same or slightly less torque and produces 28 knots at wot.
    I feel sure that if I manage to get a more laminar flow in the tunnels there will be less vibration and I will dare to open the throttle further. There will still be ventilation because the props are not submerged, but that's why they are oversized for this boat. The final step would be to improve efficiency with cupped props or cleavers.

    Cornelis Koger
     
  4. Guest625101138

    Guest625101138 Previous Member

    CDK
    I appreciate the feedback. Not many are prepared to take on a project of that scale and complexity without a lot of professional help. No doubt you have many of the requisite skills.

    I think your choices have been discussed on the other thread.

    I agree power is enough. You have to decide to duct or use a more suitable prop if you want to plane. What I do know for sure is a foil prop with low EAR is not the right prop to have working at the surface. As you have experienced, the produce high vibration and lots of white water.

    My testing simply resulted in becoming a drowned rat to the joy of onlookers apart from my son who helped me carry the boat to the water. I think silly old fool may have crossed his mind.

    Rick W
     
  5. clmanges
    Joined: Jul 2008
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    clmanges Senior Member

    Just for grins, here's the scale of what I was thinking of, and some ideas for the blade profiles. My boat drafts about seven inches, so that would be the difference between the inner and outer diameters; the O.D. would be about 16".

    I know that the blades will not have an airfoil section, so thin material, and I'm thinking they could possibly have overlapping margins.

    As to the blade profiles on the right (normal motion from right to left), I figure the flat one would of course be easiest to produce, but probably the least efficient; it would have to be mounted at 45 degrees to get the best push, and would be equally effective in forward or reverse, and the highest sideways thrust. The "Venician Blind" type might be best, but I have no way to qualify that hunch; they'd be a little less effective in reverse.

    Notice that the tips of the exponentially curved blades would enter the water at zero pitch, so would have the cleanest entry. Little to no effectiveness in reverse. Sorry, they're not very well drawn, but you can get the idea.

    Maybe this helps.

    It's a sort of slow, single-stage turbine, and maybe someone (Rick, I suppose) could guess at a velocity range needed.
     

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  6. Guest625101138

    Guest625101138 Previous Member

    Curtis
    Flat blades set at 50 degrees at the ID and 60 degrees at the OD (relative to boat axis) would be a good start. Means they would have a slight twist.

    You have to think in velocity vectors that combine the radial velocity and the boat speed. So the vector changes with radius. The angle of attack of the blades will be quite low - a couple of degrees.

    A flat plate has a better chance of aerating on the low pressure side because there will be a pronounced low pressure zone around the leading edge. Air will be dragged down this and then sheet across the front of the blade thereby reducing drag.

    Rick
     
  7. clmanges
    Joined: Jul 2008
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    clmanges Senior Member

    Yeah, I realized that a couple hours ago. I was thinking about the exponentially curved blades, then realized that a zero angle of attack would be pushing against the flow of water past the boat.

    My next thought was that the blade angles need to be optimized for the boat's speed, along with the rotational velocity of the wheel, as you've stated. Since this would be going on my slug-butt rowboat, I think its top speed is maybe two or three mph., or maybe that's cruising speed under oars; I'm not sure. What kind of rotational speed does that give?
     
  8. Guest625101138

    Guest625101138 Previous Member

    The angle of attack is measured with respect to the velocity vector. So it would only be zero if the boat was not moving. As soon as the boat moves forward the angle of attack will be negative.

    For blade angle you need to determine the velocity vectors across the radius. Two locations will be good enough. Say at the root and tip of the blade.

    Something like 60rpm would be design condition. And yes boat speed of 3mph will be close to tops. You now have enough information to determine the velocity vectors at the chosen radii. Set the blade angle about 3 degrees more than the vector at each location. This is the way a prop is pitched. Angles might be a bit larger than I have given. This will be good because there will be less side thrust.

    You will have confused flow around the transom of the boat making real life conditions somewhat different. Would pay to set the prop as far aft as practical.

    Rick W.
     
  9. clmanges
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    clmanges Senior Member

    Rick,
    Thanks, that gets me to a point where I can work on it myself for a while. I'll go through it and see what I come up with and check back with you.
    Yeah, I had that concept; I just got the wording wrong (oops).
     
  10. alexlebrit
    Joined: Aug 2006
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    Location: France - Bourbriac

    alexlebrit Senior Member

    Peristaltics?

    Rick, you've yet again thrown up another idea (where do thay all come from) and that was the peristaltic drive.

    [​IMG]

    which you reckon could get about 70% efficiency. Given that it's got to be almost totall weed-rsistant, although I suppose a big lilly pad could get sucked onto the water inlet, I'm thinking this might be a good system for a boat built for long days on weedy water. The thought of course being that whilst it might provide you with a slower speed whilst underway, if you don't have to stop every ten minutes to de-weed, your average speed and enjoyment would be much greater.

    Has anyone done any experimenting with these in PPB's (or similar). I've not seen anything on the web, only people trying to replicate squids, and this seems so much easier as it's basically a rotary motion, just like a pedal.
     
  11. Guest625101138

    Guest625101138 Previous Member

    Alex
    My concept was based around a large diameter partial loop. Or more like the linear type:
    http://en.wikipedia.org/wiki/Image:Schlauchpumpe-lineare-Bauart-transparant-indizert-bewegt.gif

    There is a lot of detail required to get 70%. It needs big bore to allow high flow rate without a lot of pressure as the velocity ratio is inverse to efficiency. Also the length of hose or ducting will cause losses in the system so important to keep them short. The water should be ejected just above water level to reduce back pressure so there is a bit of lift involved that costs. So I guess a wide flat nozzle would be best.

    I cannot recall what size hose I did calculations on but I think it was 100mm. The intention was to mount this in the bottom of the hull and have rollers supported on a spiderwheel roll over the tube. The compliance of the rubber is important otherwise energy is lost there.

    I might build one out of curiosity to see if I can solve the engineering problems but it is not in the near future.

    If you were keen to try it I could do calculations but I do not want to waste time if there is no detailed concept that looks practical.

    Rick W
     
  12. alexlebrit
    Joined: Aug 2006
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    alexlebrit Senior Member

    Hmm, now that one looks a hell of a lot more complicated than the one I've posted and is there any real benefit to all the added complication, which would surely add weight, as always there's a trade off there between efficiency of drive and efficiency of the boat you end up having to make to put it in.

    And again forgive me for possiby being stupid but is back pressure a problem, I'm not certain on the "velocity ratio is inverse to efficiency" part. I can see that too much back pressure would potentially cause leakage back past any roller, but I have a feeling you're talking of something different.

    I'm quite happy to attempt calculations myself, if you could point me in the direction of what they might be.
     
  13. Guest625101138

    Guest625101138 Previous Member

    Alex
    It just takes me time to brush out the cobwebs with these things.

    The physics involved is covered by Newton's laws.

    Force = m* dv/dt + v * dm/dt

    work is F.ds

    power is rate of doing work

    Water comes in at zero speed relative to ground and is accelerated backwards.

    The easy part is that water is incompressible so dm/dt is simply related to velocity and cross sectional area.

    For a pedal boat you need a force around 30N to make reasonable headway at say 2.5m/s. So the boat power is 30*2.5 = 75W. Now all you need to do is work out the size of ducting and velocity ratio to produce the 30N thrust from the jet and the power to produce that thrust. You then can determine efficiency. Of course the result will be the theoretical maximum efficiency for the chosen conditions. There are other losses associated with the height of outlet above the surface and the slight pressure loss in the ducting but these will be quite small compared with the theoretical limits.

    I would base calculations on a velocity ratio between inlet and outlet.

    Rick W
     
  14. Village_Idiot
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    Village_Idiot Senior Member

     

  15. clmanges
    Joined: Jul 2008
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    clmanges Senior Member

     
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